Multi Standard Radio (MSR) transmitters are used in, e.g. base stations supporting different radio access technologies such as LTE, WCDMA, GSM, etc. A schematic design of a MSR is illustrated in FIG. 1. Said MSR transmitters suffer from high PAR values, i.e. a high ratio between peak power and average power of the signal. This is a consequence of the central limit theorem and of that many signals are added together. The resulting real and imaginary components of the signal will approach Gaussian distributions and the amplitude will approach a Rayleigh distribution.
It is important to limit the PAR in order to get good efficiency of the Power Amplifier (PA). For limiting PAR a Crest Factor Reduction, CFR, algorithm may be used.
In FIG. 2 is a digital input signal illustrated wherein sample amplitude values are illustrated as dots. Some of the sample values exceed a threshold TR. A peak is herein defined by a group of samples above a certain threshold value TR. In FIG. 3, two groups of samples exceed the threshold value. The two groups are separated by a sample whose amplitude value is less than TR. The part of a sample amplitude value overshooting or exceeding a threshold is illustrated. This is the peak absolute value, i.e. the absolute value of the peak sample.
The most straightforward way would be to just clip the signal at a chosen threshold, see FIG. 4. This would however result in out of band spectrum emission, something that is not acceptable in a MSR transmitter.
In said MSR transmitter, an aggregated baseband carrier signal is generated and modulated to radio frequency before being powered up and transmitted. Said aggregated baseband carrier signal comprises carrier passbands, wherein each carrier is allocated to a carrier passband which passbands are separated by intermediate free frequency bands. Said passbands and intermediate free frequency bands are allocated according to a carrier passband configuration in the baseband. An example of such a carrier passband configuration is illustrated in FIG. 5. One condition for a MSR transmitter is that said intermediate free frequency bands should be free and clean from frequency components and signal energy generated by the transmitter. Thus, the transmitter should not generate signals outside its transmission band.
A useful CFR algorithm must therefore be able to limit the PAR and limit the out of band emission without too much increase of the Error Vector Magnitude, EVM, which is a measure of the distortion of the signal.
Peak Cancellation is one of the most common CFR algorithm used today for reducing a peak in a signal by adding a band limited pulse to cancel the peak. Only the highest sample in the peak is used for determining the amplitude and phase of the subtracted pulse.
Peak cancellation comprises three stages: peak extraction, peak detection and peak cancellation.
The purpose of the peak extraction is to identify all samples that have an absolute value above the threshold value. For Peak Cancellation it is also necessary to identify the largest sample in each group of samples above the threshold value. This is called Peak Detection. This sample is the only value that is used for the peak cancellation.
There are a two classical peak detectors used for Peak Cancellation CFR. The simplest one looks for the first sample in the interval over the threshold value that is larger than its neighbors. The drawback with this method is that it there is a risk that a local maximum is detected as the largest sample.
A better but more complex method is to actually find the largest sample in the interval. As soon as the interval has ended, i.e. one sample is below the threshold, the algorithm selects the so far largest sample.
In case of several carriers as in a MSR transmitter, a set of narrow peaks occur within a very short time range, as illustrated in FIG. 7. FIG. 7 is a graph diagram illustrating the amplitude of peaks over a threshold value TR as a function of time. Each dot is a sample. Within a time interval, nine narrow peaks of samples exceeding the threshold value TR occur. The classical peak detector would in this case identify each peak and the algorithm would try to cancel each peak separately. This will be very inefficient.
If each narrow peak is detected separately the number of peaks will be too high to handle.